1. Field of the Invention
The present invention relates to a substrate, a conductive substrate, a fine structure substrate, an organic field effect transistor and a method of manufacturing them, in particular to a substrate, a conductive substrate and a method of manufacturing them in which variously applicable layers such as electrodes, metal wiring, insulating layers, and fine patterns of these layers can be formed on a base material having an unevenness such as resin material and in particular to a substrate, a conductive substrate, a fine structure substrate, an organic field effect transistor, and a method of manufacturing them in which a flat and smooth layer can be provided by a simple method such as coating on an inexpensive base material to form layers such as electrodes, metal wiring and an insulating layer easily with good adhesion and which can be used for making fine elements such as semiconductor elements, integrated circuits, image display device and the like.
2. Related Background Art
In recent years, under the situation that compactness, thinness and portability of electronic devices are given importance, attention is being paid to electronic parts in which thin and flexible base material such as resin material are used. At the same time, finer structure, higher density, high reliability, cost reduction are also being required, thus substrates appropriate for manufacturing electronic parts are becoming indispensable. In addition, a fine structure substrate having a fine structure such as metal wiring formed on a resin substrate is being highly desired.
Furthermore, since the latter half of the 1980s, field effect transistors using organic semiconductor materials have gradually been developed actively, and have entered such a stage as to surpass the characteristics of thin-film transistors of amorphous silicon as basic performance. Because organic materials can easily be processed and generally have a high affinity to plastic substrates for formation of thin film FETs (Field Effect Transistors), and therefore because organic materials enable manufacturers to adopt low-temperature processing, transistors can be formed so as to have a large area at low cost with a simple process by use of manufacturing methods such as coating and printing. Drive circuits for thin film displays and electronic papers, radio frequency identification (RFID) tag and IC cards and the like are being assumed for application and development, and there exist a number of technical reviews (for example, C. D. Dimitrakopoulos et. al, “Organic Thin Film Transistors for Large Area Electronics,” Advanced Material, 14, 2002, No. 2, P. 99-117).
In particular, organic field effect transistors on resin substrates have all the foregoing advantages and are industrially very useful. Accordingly, provision of organic field effect transistors having a fine structure such as a metal wiring or insulator formed on a resin substrate are highly desired.
Hitherto, for example, in the case of manufacturing electric/electronic circuit substrates such as semiconductor integrated circuits (IC) or electronic parts such as capacitors for DRAMs, as a method of forming a fine structure at a predetermined part on a substrate, the method has been known in which a layer is prepared on a substrate by means of a gas phase method such as evaporation, chemical vapor deposition (CVD), sputtering, or a coating method using sol-gel process or the like, a pattern is formed using photolithography, and thereafter unnecessary portions are removed.
However, in the case of forming a fine structure substrate by means of the photolithography, a vacuum apparatus for evaporation, sputtering, etc., is used to form a thin film, which is then pattered to produce a device. In general, with the photolithography, patterning is effected through the following steps. At first, a thin film to be patterned is formed entirely on a surface of a substrate. Moreover, resist coating, exposure, development, rinsing and so on are performed to form a resist pattern. Thereafter, the resist pattern is used as a mask to effect etching to remove an unnecessary portion to provide to a desired pattern shape. As described above, the photolithography method requires very large number of steps. Therefore, the photolithography method is costly.
On the other hand, as methods of forming electrodes on electric/electronic circuit substrates such as semiconductor integrated circuit (IC) or the like, simple methods has become available which utilizes an ink jet method or a coating method such as screen printing or offset printing using metal fine powder-dispersed paste or slurry. It is expected that this will enable metal electrodes to be formed without using a complicated method such as plating, vacuum evaporation, sputtering, etc., or a costly method such as photolithography method and the like.
However, in the case of a printing method such as screen printing, for example, formation of a fine pattern with a line width of 10 μm or less entails difficulties. The reason is that the processing accuracy of the mask is reflected to the pattern accuracy.
On the other hand, such printable electrode material is becoming capable of obtaining a high conductivity similar to that of an evaporated metal film, which poses the large problem that a calcination temperature of 150° C. or more will be required in order to attain such a high conductivity. Therefore, the heat resistance of the substrate material or substrate is nonnegligible. Polyethylene terephthalate (PET) film, which is the most general-purpose resin base material at present, is inferior in terms of heat resistance and is therefore difficult to be used when heated, e.g., to 150° C. or more. On the other hand, polyethylene naphthalate (PEN) and polyimide base materials withstand heating up to about 200° C. but is expensive compared with PET.
In addition, in the case of forming an electrode pattern using liquid through printing or the like, the surface state of the base material affects the characteristics of the obtained film, which results in the possibility that no desired conductivity may be obtained or necessitates that the film is made thicker than needed. The surface state of the base material includes, for example, the flatness and smoothness of the surface. Although a resin base material formed in a seat shape has the advantages of high processability and deformability as well as inexpensiveness, a number of steps or unevennesses generated at the time of forming are recognizable and the surface flatness and smoothness are inferior to those of glass or silicon base material, which has been a problem. In the case where a solution was coated on such resin base material surface with the aid of a bar coater and dipping, uniform coating has been difficult. The film characteristics after the calcination have not also been uniform.
Further, in a conductive ink or the like, the wettability will vary largely depending on the state of the base material surface. In particular, in case of a water dispersion, on a base material having a large water contact angle, there is a possibility that shedding may occur, or the uniformity of film thickness may remarkably lower, or particles in the dispersion may aggregate. Moreover, also in case of using pastes to form a pattern, the pattern shape and the resolution is largely affected by the unevenness or wettability of the base material surface.
The wettability of the base material surface can be improved with a variety of surface modification methods. Those methods include chemical treatment methods using chemical agents or coupling agents and physical treatment methods including ultraviolet radiation and plasma contact, etc. The former chemical treatment method is exemplified by a method of immersing a base material in sulfuric acid to make the surface hydrophilic, which, however, may cause a serious damage depending on the kind of the base material and necessitates post-treatment cleaning. On the other hand, the ultraviolet radiation and plasma treatment have been known to have an effect of making hydrophilic a variety of resin base materials including fluororesin base materials and improve the adhesion. However, because the effect of surface modification by the treatment will vary depending on the kinds of base materials, the applicable base materials are limited, which has been a problem.
Under the above circumstances, an example has been reported in which a hydrophilic polymer thin film is formed on a base material to effect surface modification. In this example, a thin film of a polymer containing a hydrophilic group such as sulfonic acid is used. Because the polymer thin film is fixed on the substrate material, irradiation with ionizing radiation is required. When a thick film is to be obtained or the fixing operation is omitted, there has been a possibility that swelling may occur at the time of the immersion in water (Japanese Patent Application Laid-Open No. H11-12376).
On the other hand, another example has been reported in which a hydrophobic film such as a silane derivative is formed on a substrate and irradiated with light through a photomask to form a hydrophilic latent image, and a metal oxide such as TiO2 is deposited on this hydrophilic latent image by sol-gel process or the like, thereby forming a pattern (for example, Japanese Patent Application Laid-Open No. 2002-169303). Because TiO2 will not grow on the hydrophobic film, a fine pattern can be obtained. However, although the film formation of TiO2 is performed at ordinary temperature, long-time ultraviolet irradiation is required in order to decrease the water contact angle to lengthen the manufacturing time, so that much cost reduction cannot not be accomplished.
Moreover, still another example has been reported in which polarities on a substrate are controlled to form a pattern. In this method, a metal oxide such as TiO2, which gives rise to light polarity change, is formed on a substrate surface and irradiated with light through a photomask to form a latent image, and an ink is deposited on this latent image using an ink jet drawing method to form a pattern (for example, Japanese Patent Application Laid-Open No. 2003-59940).
Because a resin substrate surface have many bumps with heights of about 200 nm, in order to fill these bumps, a film of not less than 2 μm in thickness needs to be formed on the resin substrate. However, because in general a metal oxide cannot be formed into a film in a thickness of a micrometer order and the bumps of the resin substrate cannot therefore be filled, no resin substrate of good smoothness will not be obtained. Moreover, the obtained pattern will suffer from blurring (staining), so that no fine pattern can be obtained. In addition, in the method, the metal oxide is formed into a film by use of the evaporation or the sol-gel process. In the case of the evaporation, migration may occur between the film and the resin substrate or the resin substrate may deform. In the case of the sol-gel process, in order to obtain an anatase-type crystal with good photosensitivity, a high-temperature calcination above 200° C. is required, giving rise to the above-mentioned problem of unemployability of PET or the like, accompanied by the problem that the available resin substrates are limited.
Yet another example has been reported in which such metal oxides are not used and substrate polarity change is utilized. With this method, a polymer compound susceptible to polarity change is grown by graft polymerization on a resin substrate subjected to reaction activation treatment or roughening treatment (for example, Japanese Patent Application Laid-Open No. 2003-76004). In this case, although the polymer compound can be grown at a low temperature, the resin substrate surface is rough, so that no fine pattern can be formed, which is a problem.
In addition, in these organic field effect transistors, a gate electrode is formed on an organic semiconductor layer. In general, a surface of an organic semiconductor material will be discomposed with UV light or the like, and therefore in the case of such stacking order, changing the surface energy by use of UV light may result in lowering in the performance. In addition, because source and drain electrodes are formed on a substrate, an insulating film on the source and drain electrodes will be thin, thereby posing the problem that a leak current is apt to be generated.
As described above, in the case of forming layers such as electrodes on a general-purpose resin base material, the existence of unevenness on the base material has resulted in difficulty in forming a desired layer such as electrode with a low resistance and uniform characteristics by use of a dispersion liquid or paste of conductive fine particles.